A Nanopower Biopotential Lowpass Filter Using Subthreshold Current-Reuse Biquads With Bulk Effect Self-Neutralization

A nanopower CMOS 4th-order lowpass filter suitable for biomedical applications is presented. The filter is formed by cascading two types of subthreshold current-reuse biquadratic cell. Each proposed cell is capable of neutralizing the bulk effect that induces the passband attenuation. The nearly 0-dB passband gain can thus be maintained, while the entire filter circuit remains compact and power-efficient. Designed for electrocardiogram detection as an example of application, the filter prototype has been fabricated in a 0.35 μm CMOS process occupying 269 μm × 383 μm chip area. Measurements verify that the filter can operate from a 1.5-V single supply and consumes 5.25 nW, while providing a cutoff frequency of 100 Hz and input-referred noise of 39.38 μVrms. The intermodulation-free dynamic range of 51.48 dB is obtained from a two-tone test of 50 and 60 Hz input frequencies. Compared with state-of-the-art nanopower lowpass filters using the most relevant and reasonable figure of merit, the proposed filter ranks the best.